The present invention relates to a method for manufacturing a semiconductor device, more specifically, a method for manufacturing a thin-film transistor.
The manufacturing process of a thin-film transistor serving as a semiconductor device includes, for example, forming a semiconductor film that is later to become a channel of the transistor, and crystallizing the semiconductor film by irradiating light (especially a laser beam). Such steps, which are performed in different manufacturing devices, involve the following problem. If a substrate is exposed to the atmosphere while the substrate is carried from one step to another, impurities in the atmosphere such as hydrocarbon, boron, phosphorus, water, etc. bond to the surface of the semiconductor film and, with the melting and solidification of the semiconductor film due to crystallization, a large quantity of impurity elements are mixed into the semiconductor film.
Conventionally, as a solution to the above problem, a technique wherein a thin amorphous semiconductor film is deposited and then a gate oxide film is formed by means of solid-phase growth with continuous heating of the thin amorphous semiconductor film without exposing it to the atmosphere has been proposed in a first related art example, which will be described later.
Further, for the purpose of preventing the mixing of impurity elements into a semiconductor film, another technique for carrying a specimen, avoiding exposure to the atmosphere, between a chamber for heating an amorphous semiconductor at a temperature lower than the crystallization temperature of the same semiconductor and another chamber for irradiating a laser beam to a semiconductor has been proposed in a second related art example, which will be described later.
In addition, a thin-film transistor manufactured based on the manufacturing process of a thin-film transistor involves, in some cases, a problem of the shift of flat-band voltage that depends on a fixed electric charge, etc. in a substrate protective film and a gate insulation film. As a solution to such a problem, a method for manufacturing a thin-film transistor that can control the variation in threshold voltage by doping boron, etc. to a channel region and fixing the flat-band voltage to 0 V, virtually, has been proposed in a third related art example, which will be described later.
As techniques that can prevent the mixing of impurity elements by avoiding exposure to the atmosphere and, at the same time, control the variation in flat-band voltage, two combinations of the above techniques can be considered, for example.
FIGS. 5A to 5C show a first combination wherein semiconductor film formation, laser crystallization, and impurity doping are performed in the described order. As shown in FIG. 5A, a semiconductor film 202 is formed on a substrate 200 through the intermediary of a substrate protective film 201 using a CVD device. Then, as shown in FIG. 5B, a crystallized semiconductor film 203 is formed by irradiating a laser beam using a laser crystallization device. Lastly, as shown in FIG. 5C, boron B+ is doped on the crystallized semiconductor film 203 using a doping device.
FIGS. 6A to 6C show a second combination wherein semiconductor film formation, impurity doping, and laser crystallization are performed in the described order. As shown in FIG. 6A, a semiconductor film 202 is formed using a CVD device. Then, as shown in FIG. 6B, boron B+ is doped on the semiconductor film 202 using a doping device. Lastly, as shown in FIG. 6C, the semiconductor film 202 is crystallized by irradiating a laser beam.
In the first combination, a heating process for activating the doped impurities is required separately. However, since the highest annealing temperature of a glass substrate is limited, the above process is replaced by the increase of the doping quantity. On the other hand, when the doping quantity is increased, the properties of a semiconductor film may be degraded. Hence, the second combination, which can achieve a highly efficient activation of impurities in the laser crystallization step, is superior to the first combination.
Japanese Unexamined Patent Publication No. 3-289140 is a first example of related art.
Japanese Unexamined Patent Publication No. 6-342757 is a second example of related art.
Japanese Unexamined Patent Publication No. 2003-257992 is a third example of related art.
In both the first and second combinations, however, all of the CVD device, doping device, and laser crystallization device need to be used in combination in order to achieve the purpose of preventing the mixing of impurity elements by avoiding exposure to the atmosphere and, at the same time, controlling the variation in flat-band voltage.
The more the number of manufacturing devices increases, the more the number of steps required for carrying a substrate to another manufacturing device increases, which leads to a cost increase. Especially, the three manufacturing devices need to be coupled with one another under an airtight state so as not to expose a substrate to the atmosphere, which makes the cost increase of manufacturing devices unavoidable.